1. Technical Field
The present invention relates to a silica glass composition and to a method for preparing silica glass by a sol-gel process using the silica glass composition.
2. Related Art
Generally, silica glass is transparent, is chemically inactive, has excellent characteristics (such as thermal stability or strength) and has a low thermal expansion coefficient. Such characteristics allow silica glass to be effective and useful for optical devices, such as optical fiber or optical lenses.
An optical fiber basically comprises of a core and a cladding having a refractive index different from that of the core so that total reflection of light occurs at the core. In connection with the total reflection of light, the refractive index of the cladding is about 1% lower than that of the core. A core having a refractive index of 1.47 and a cladding having a refractive index of 1.46 are generally used.
To manufacture an optical fiber, an optical fiber preform comprised of a core rod and an overcladding tube surrounding the core rod is first fabricated. Then, the optical fiber preform is heat-treated and elongated to form the optical fiber.
A method for manufacturing an overcladding tube formed of silica glass using a sol-gel process will now be briefly described.
First, silica particles are mixed with deionized water and a dispersant. Then, a binder and a plasticizer are added to the mixture and are mixed sufficiently to form a sol.
The sol is subjected to an aging-treatment for a predetermined time. Air bubbles are removed from the aging-treated sol. A gelling agent is added thereto, and the resultant mixture is then poured into a mold.
When gelation is completed, the resultant gel is removed from the mold and is then dried.
Thereafter, the dried gel is heat-treated to remove organic substances in the gel. Subsequently, dehydroxylation and sintering processes are performed on the resulting gel, thereby obtaining an overcladding tube formed of silica glass.
Fumed silica is used as the silica particles. The fumed silica is pyrogenetic silica produced by pyrolyzing a silicon compound (e.g., SiCl.sub.4), and has a specific surface area of 50.about.400 m.sup.2 /g and an average particle diameter of 5.times.10.sup.-2 .mu.m.
However, in the case of manufacturing silica glass using the fumed silica as the silica particle, the following problem may be generated.
Since the fumed silica is a microparticle having a small pore, it is difficult to disperse a large amount of fumed silica into the sol. Also, much time is required to remove air bubbles from the sol, and the viscosity of the sol increases excessively. Due to such characteristics of fumed silica, it is difficult to increase the content of silica solid in the sol to 50% or higher. Thus, the green density of the resultant, obtained by removing the organic substance in the gel, is limited to about 34% based on the density of the sintered body, which increases the shrinking ratio of the overcladding tube before and after the sintering process. Thus, it is practically difficult to manufacture a large silica glass tube. Also, a crack may occur in the overcladding tube after it is dried.